The present invention relates to methods and apparatus for use in wireless (mobile) telecommunications systems. In particular, embodiments of the invention relate to methods and apparatus for providing coverage extension in wireless telecommunications systems.
Third and fourth generation mobile telecommunication systems, such as those based on the 3GPP defined UMTS and Long Term Evolution (LTE) architecture are becoming able to support more sophisticated services than simple voice and messaging services offered by previous generations of mobile telecommunication systems. For example, with the improved radio interface and enhanced data rates provided by LTE systems, a user is able to enjoy high data rate applications such as mobile video streaming and mobile video conferencing that would previously only have been available via a fixed line data connection. The demand to deploy third and fourth generation networks is therefore strong and there is a corresponding desire to extend the coverage available in such telecommunications systems (i.e. there is a desire to provide more reliable access to wireless telecommunications systems for terminal devices operating in coverage-limited locations).
A typical example of a coverage-limited terminal device might be a so-called machine type communication (MTC) device, such as a smart meter located in a customer's house and periodically transmitting information back to a central MTC server relating to the customer's consumption of a utility, such as gas, water, electricity and so on. Such a terminal device might operate in a coverage-limited location because, for example, it may be located in a basement or other location with relatively high penetration loss.
In some situations a terminal device in a coverage-limited situation in a particular communication cell served by a base station might be unable to receive communications from the base station unless specific provision is made for it to do so. One simple way to increase coverage in this situation would be for the base station to increase the power of its transmissions. However, a blanket increase in transmission power from a base station would be expected to give rise to correspondingly increased interference in neighbouring communication cells. An alternative approach would be for the base station to in effect focus/concentrate its available transmission power budget into a subset of transmission resources (e.g. in terms of frequency) which are selected from within the base station's overall transmission resources and allocated for transmissions to coverage-limited terminal devices. In this manner increased power may be made available for communicating with terminal devices in “hard to reach” locations without exceeding a base station's overall power budget. Such an approach may be referred to as power boosting. Thus, a base station with power boosting capability may focus its available transmission power within a restricted subset of transmission resources allocated to coverage-limited terminal devices.
This power boosting approach is schematically represented in FIGS. 1A and 1B which show example plots of maximum allowed transmission power P versus frequency f for two modes of operation for a base station in an LTE-based wireless telecommunication network. FIG. 1A represents a normal mode of operation in which the maximum allowed transmission power is uniform across the base station's full operating bandwidth SBW (e.g. 20 MHz) at a level of P0. FIG. 1B, on the other hand, represents a power boosted mode of operation for the base station in which the overall available transmission power is in effect concentrated with transmissions being allowed at a power level PPB, which is greater than the power P0 for the normal operating mode, across a bandwidth PBBW, which is less than the bandwidth SBW for the normal operating mode. It can be expected a base station will be adapted to switch between normal and power boosted operating modes, for example depending on current or expected traffic conditions. The overall transmission power will typically be broadly the same in both operating modes (i.e. the areas under the curves in FIGS. 1A and 1B will be the same). For the sake of a concrete example, in one power boosted operating mode implementation PBBW may be approximately one quarter of SBW (e.g. SBW=20 MHz and PBBW=5 MHz) while PPB may be approximately four times P0. Thus, in this example implementation the base station may transmit up to four times more power on transmission resources allocated to a coverage-limited terminal device within the frequency bandwidth PBW without exceeding an overall power budget for the base station. In practice, communications with specific terminal devices on specific subcarriers may be made with less power than the maximum allowed, taking into account the conventional power control mechanisms provided in wireless telecommunications systems.
Thus, a wireless telecommunications network adapted to provide coverage in challenging situations by power boosting may at times re-configure itself to concentrate its available transmit power into a number of resource elements (REs) occupying in total less than the nominal system bandwidth. A coverage-limited terminal device may be allocated resources on these power-boosted resource elements making it more likely to be able to use the cell.
As is well understood, in an LTE type network there are two Radio Resource Control (RRC) modes for terminal devices, namely: (i) RRC idle mode (RRC_IDLE); and (ii) RRC connected mode (RRC_CONNECTED). When a terminal device transmits data, RRC connected mode is required. In RRC idle mode, the core network (CN) part of the wireless telecommunications system recognizes the terminal device is present within the network, but the radio access network (RAN) part of the wireless telecommunications system does not. As is conventional for an LTE-type wireless telecommunications network, a terminal device may conduct Reference Signal Received Power (RSRP)/Reference Signal Received Quality (RSRQ) measurements in communication cells in which it can operate and may autonomously decide to camp on one particular cell (e.g. according to RSRP/RSRQ threshold tests and the Public Land Mobile Network (PLMN) identities of the cells) in order to receive system information (SI) and paging messages. In accordance with this approach the base stations supporting communications in the respective cells do not themselves play a role in cell selection for terminal devices in idle mode with the process of cell selection/reselection in idle mode being performed autonomously by the terminal devices. This is in contrast to the cell-change procedures in RRC connected mode in which case terminal devices are under control of the RAN and the handover process is a network controlled behaviour (with assistance from terminal device measurements).
A terminal device that could benefit from power boosting as described above to more reliably operate in a communication cell will typically not know at the point of trying to acquire or camp on a cell whether the base station of the cell supports power-boosting. As a consequence, a terminal device may spend time and power resources undertaking a camp on procedure for a cell, for example by decoding Primary Synchronisation Signalling (PSS), Secondary Synchronisation Signalling (SSS), a Physical Broadcast Channel (PBCH) and SI of a cell, and then subsequently undertake a random access procedure using Physical Random Access Channel (PRACH) resources to access the cell, only to find the cell does not support power boosting and so cannot reliably support communications with the terminal device on channels such as a Physical Downlink Control Channel (PDCCH) and Physical Downlink Shared Channel (PDSCH).
Even for a base station which is able to support power boosting, it may be that the above-discussed power boosting approach to extending coverage may not be supported by the base station at all times so as to reduce the impact on other terminal devices operating in the cell. For example, power boosting may only be supported at certain times of day or night within a given communication cell according to when it is expected the resources required to properly support conventional terminal devices operating in the cell may be reduced. In these cases it may be appropriate for terminal devices requiring power boosting to wait (“sleep”) until such time that power boosting is supported before seeking to acquire the relevant cell.
There is therefore a need for schemes which assist in the process by which a terminal device which may benefit from power boosting for reliable communications in a wireless telecommunications system seeks to camp on/access base stations of the wireless telecommunications system.